High performance linear motor and magnet assembly therefor

ABSTRACT

A magnet assembly includes a back iron and an array of magnets. The back iron is in the form of a plate having opposed surfaces. The magnets are arranged along one of the surfaces, with the other surface being dimensioned and configured according to the magnetic field distribution associated with the magnets. The back iron geometry provides for reduced mass, reduced leakage flux, and high flux densities to improve performance of a linear motor that employs such a magnet assembly.

REFERENCE TO RELATED APPLICATIONS

This is a divisional application of U.S. patent application Ser. No.10/369,161, filed Feb. 19, 2003 now U.S. Pat. No. 6,803,682, entitledHIGH PERFORMANCE LINEAR MOTOR AND MAGNET ASSEMBLY THEREFOR which claimsthe benefit of U.S. Provisional Patent Application Ser. No. 60/358,654,filed on Feb. 21, 2002, and entitled HIGH PERFORMANCE LINEAR MOTORMAGNET ASSEMBLY THEREFOR. The entireties of these applications areincorporated by reference herein.

TECHNICAL FIELD

The present invention relates to motors and, more particularly, to amagnet assembly and to a linear motor incorporating the magnet assembly.

BACKGROUND OF THE INVENTION

Linear motors are used in various types of systems, such as forpositioning and moving applications, including machining and gantry typesystems. The high performance systems often require moving elementssubjected to high acceleration levels. In order to achieve such highacceleration, the linear motor must exert large forces upon the elementsto be moved.

There are various configurations of linear motors, including flatmotors, U-channel motors and tubular shaped motors. Different types oflinear motors are also available, including brush, AC brushless,stepper, and induction motors. Common to most linear motors is a movingassembly, usually called a forcer or stage, which moves relative to astationary platen (or path) according to magnetic fields generated byapplication of current through one or more associated windings. Thewindings can be on the forcer or at the platen depending on the type ofmotor. For example, in a permanent magnet linear motor, a series ofarmature windings can be mounted within a forcer that is movablerelative a stationary path. The path can include an array of permanentmagnets configured to interact with the coils in the stage whenenergized with an excitation current.

Alternatively, in another type of conventional linear motor, permanentmagnets can be part of a moveable stage with the coils situated in theplaten. Usually, the permanent magnets are attached to a back iron plateabove the coils, which are oriented along a path of travel. The magnetsusually are rectangular in shape. The magnets are arranged along theback iron so that adjacent pairs of magnets have opposite magnetic poleorientations. The magnets can be oriented generally normal to thedirection of travel or inclined at a slight angle from normal to an axisof the direction of travel for the linear motor. The inclined anglecreates a flux distribution along the axis of movement which isgenerally sinusoidal in nature. Such a resulting distribution due to theoptimized motor geometry tends to reduce cogging during operation of thelinear motor, which would otherwise occur if the magnets were aligned,normal to the axis of movement.

Although an inclined angle of the magnets can reduce some cogging, itpresents a disadvantage in that a larger area typically must be coveredby the rectangular magnets in order to sufficiently cover and interactwith the coils of the armature. When the magnets are implemented with alarger area so as to reduce cogging effects, a larger footprint for theback iron also is required. This tends to increase the overall weightand size of the stage. Such increases in size and weight can presentadditional obstacles, such as in applications were there are sizeconstraints and low mass is desirable. For example, as the mass of thestage increases, the available acceleration experiences a correspondingreduction, and the ability to stop the motor accurately also reducesbecause of the increased power dissipation needed to stop the motor.

As the use of linear motors in manufacturing equipment continues toincrease, nominal increases in the speed of operation translate intosignificant savings in the cost of production. Accordingly, it isdesirable to provide a magnet assembly that can be part of a highperformance linear motor.

SUMMARY

The following presents a simplified summary of the invention in order toprovide a basic understanding of some aspects of the invention. Thissummary is not an extensive overview of the invention. It is intended toneither identify key or critical elements of the invention nor delineatethe scope of the invention. Its sole purpose is to present some conceptsof the invention in a simplified form as a prelude to the more detaileddescription that is presented later.

One aspect of the present invention provides a magnet assembly that canbe employed as part of a linear motor stage to form a high performancelinear motor. The magnet assembly includes a plurality of magnetsoperatively associated with magnetically conductive plate, commonlyknown as a back iron. The magnets extend from a common side of the backiron. The back iron is dimensioned and configured to substantiallyconform to magnetic flux that travels through the back iron when themagnet assembly is exposed to a magnetic field, such as from windings ofa motor path. In one particular aspect of the present invention, across-sectional dimension of the back iron varies between opposed endsof the back iron as a function of the position and/or orientation of themagnets. For example, a thickness of the back iron is greater atlocations between adjacent pairs of the magnets than at locationsgenerally centered with the respective magnets. As a result of such backiron geometry, force output to moving mass ratio of a motorincorporating the magnet assembly is improved over conventionalconfigurations of magnet assemblies. Also, the back iron geometryreduces leakage flux.

Another aspect of the present invention provides a linear motor systemthat includes a path having a plurality of windings, which can beenergized to produce desired magnetic fields. The linear motor systemalso includes a magnet assembly, such as described above. The linearmotor system achieves high performance because the magnet assembly has areduced mass, which substantially conforms to magnet flux lines thattravel through the magnet assembly during energization of path windings.The mass further can be reduced by employing generally elongatedoctagonal magnets, such as by removing corner portions from rectangularmagnets.

To the accomplishment of the foregoing and related ends, certainillustrative aspects of the invention are described herein in connectionwith the following description and the annexed drawings. These aspectsare indicative, however, of but a few of the various ways in which theprinciples of the invention may be employed and the present invention isintended to include all such aspects and their equivalents. Otheradvantages and novel features of the invention will become apparent fromthe following detailed description of the invention when considered inconjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view of a moving magnet assembly in accordancewith an aspect of the present invention.

FIG. 2 is a top elevation of the magnet assembly of FIG. 1.

FIG. 3 is a side sectional view of the magnet assembly taken along line3—3 of FIG. 2.

FIG. 4 is side sectional view of part of a linear motor in accordancewith an aspect of the present invention.

FIG. 5 is a side sectional view similar to FIG. 4, illustrating magneticflux lines for an energized linear motor in accordance with an aspect ofthe present invention.

FIG. 6 is a side sectional view of a motor magnet assembly in accordancewith another aspect of the present invention.

FIG. 7 is a side sectional view of a motor magnet assembly in accordancewith another aspect of the present invention.

FIG. 8 is a side sectional view of a motor magnet assembly in accordancewith another aspect of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a magnet assembly for use in a linearmotor. The magnet assembly includes a back iron and an array of magnets.The back iron is in the form of a plate having opposed surfaces of theback iron. The magnets are arranged in a generally linear array alongone of the surfaces. The other surface of the back iron plate isdimensioned and configured according to the magnetic field distributionand/or localized regions of saturation associated with the motorgeometry/topology. For example, the surface of the back iron plateopposite to which the magnets are attached can be scalloped, such that adimension between the opposed surfaces at locations generally alignedwith the magnet centers is less than a dimension between the opposedsurfaces at locations between adjacent magnets.

FIGS. 1, 2 and 3 illustrate a magnet assembly 10 in accordance with anaspect of the present invention. The magnet assembly 10 includes anarray of permanent magnets 12, 14, 16, 18, 20, 22, 24, and 26 ofalternating magnetic polarity (see FIG. 2). The magnets 12-26 arearranged in a substantially parallel relationship to each other andmounted to a generally rigid and magnetically conductive plate 30,commonly referred to (and hereinafter referred to) as a back iron. Thealternating polarity facilitates the flow of magnetic flux through themagnets 12-26 and the back iron 30. The assembly 10 also includes anouter encapsulation 32 of a suitable non-conducting material, such as anepoxy or a polymer material. The encapsulation 32 helps hold the magnets12-26 and back iron 30 in a desired relationship.

FIG. 2 is a top elevation of the magnet assembly of FIG. 1 in which theencapsulation 32 has been removed. As shown in FIG. 2, the magnets 12-26have a generally rectangular geometry and are spaced apart from eachother by a predetermined distance. The magnets 12-26 have long axes,which are oriented generally perpendicular to a desired direction oftravel for the assembly 10, indicated at 34, and which are alignedsubstantially parallel to each other. To provide desired fluxdistribution, the corners of each of the magnets 12-26 have beenchamfered to form magnets having elongated octagonal geometries, such asshown in FIG. 2. The precise configuration can vary depending on thesize of the magnets 12-26, the size of the motor in which the assemblyis to be employed as well as the desired characteristics for the motor.In this example, the illustrated magnet geometry also helps reduce themass of the magnet assembly 10. By way of example, the magnets areformed of a NdFeB material or other type of high performance permanentmagnetic materials.

Referring to the side-sectional view of FIG. 3, the 12-26 magnets aremounted to and extend from a common side 36 of the back iron 30. Theback iron 30 also includes another side 38 opposite the side 36 to whichthe magnets 12-26 are mounted. In particular, the magnets 12-26 arepositioned in slots or receptacles on the side 36, which are dimensionedand configured to receive a portion of the respective magnets therein.Adjacent pairs of the slots define notches 40, 42, 44, 46, 48, 50, and52 of the back iron material that extend between adjacent pairs ofmagnets. The notches 40-52 operate to separate adjacent pairs of themagnets 12-26 by a predetermined distance, indicated at 56, whichcorresponds to the width of the respective notches. For example, lessthan one-half the width of the magnets 12-26 are recessed into the backiron 30, such that more than one-half the width of the magnets extendoutwardly from the side 36 of the back iron 30. The notches 40-52 andremaining surface of the side 36 are generally coplanar, although othershapes and configurations could be used in accordance with an aspect ofthe present invention. Also, the notches 40-52 act as retainers lockingthe magnets in place providing the desired stiffness.

In accordance with an aspect of the present invention, a cross-sectionaldimension of the back iron 30 varies along its length between spacedapart ends 58 and 60 so as to substantially conform to the magnetic fluxgenerated during operation of a motor that includes the magnet assembly10. In the example of FIG. 3, the thickness of the back iron 30 betweenthe opposed sides 36 and 38 is greater at locations between adjacentpairs of the magnets 12-26 than at locations generally aligned withcenters of the respective magnets. The back iron 30 can be formed ofsubstantially any generally rigid material capable of conducting amagnetic field, so as to help form a magnetic circuit formed of themagnets 12-26 of different polarities and associated motor windings (notshown).

For example, the back iron 30 is formed of a non-linear material havinga high magnetic permeability and desired saturation characteristics. Ina particular aspect of the present invention, the back iron is formed ofvanadium permeadur (e.g., cobalt—48.75%, Vanadium—2%, Carbon—0.004%,Manganese—0.05%, Silicon—0.05%, Iron—balance), which has particularlyhigh saturation characteristics compared to other non-linear materials.While such material is considerably more expensive than steel, itssuperior magnetic properties are desirable in ultra-high performancemotors according to the present invention. It is to be understood andappreciated that a high performance magnet assembly, in accordance withan aspect of the present invention, could employ other types ofnon-linear materials (e.g., M19 steel) than vanadium permeadur.

By way of illustration, the back iron 30 has a maximum thickness,indicated at 62, at its ends 40 and 42 and at locations 64, 66, 68, 70,72, 74, and 76 between adjacent pairs of magnets 12-26. In the exampleof FIG. 3, the locations 64-76 having the maximum thickness 62 aresubstantially coextensive with the notches 40-52. Additionally, the side38 of the back iron 30 at the ends 58 and 60 and at the locations 64-76are generally coplanar and substantially parallel to the other side 36of the back iron. It is to be appreciated, however, that back iron othershapes (e.g., curved in the direction of travel) also could be utilizedin accordance with an aspect of the present invention. Thus, as shown inFIGS. 1 and 2, the side 38 defines generally rectangular and coplanarstrips extend between side edges 80 and 82 of the back iron 30 at theends 58 and 60 and at the locations 64-76.

The back iron 30 further has a minimum thickness, indicated at 84, atlocations 86, 88, 90, 92, 94, 96, 98, and 100 substantially centeredwith the long axes of the respective magnets 12-26. In the example ofFIG. 3, the locations 86-100 have the minimum thickness 84, which definegenerally rectangular planes or strips in the side surface 38 spacedfrom and substantially parallel to the magnets 12-26 over which therespective locations are positioned. The locations generally rectangularstrips, which can be coplanar, extend between the side edges 80 and 82of the back iron 30.

The portions of the side 38 extending between the locations of maximumthickness (e.g., the ends 58 and 60 and the locations 64-76) and thelocations of minimum thickness 86-100 slope upwardly and downwardly toprovide a desired scalloped or sawtooth cross section, as illustrated inFIG. 3. That is, the locations (or strips) 64-76 and 86-100 respectivelyprovide alternating peaks and valleys along the surface 38 of the backiron.

Referring to FIG. 2, each of the locations 64-76 of maximum back ironthickness has a width 104 in the direction 34, which width is greaterthan or equal to zero. Similarly, each of the locations 86-100 ofminimum back iron thickness has a width 106 in the direction 34, whichwidth is greater than or equal to zero. Accordingly, while the locationsof maximum and minimum thickness are illustrated as generally planar andparallel to the side 36, those skilled in the art will understand andappreciated that virtually any widths 104 and 106 can be employed toprovide different varying cross-sectional configurations for the backiron in accordance with an aspect of the present invention. Additionallyor alternatively, while the locations 64-76, the locations 86-100 andthe portion of the side surface extending therebetween are illustratedas generally planar surfaces, it is to be appreciated that one or moreof such surface portions could be curved in accordance with an aspect ofthe present invention.

FIG. 4 illustrates a cross-sectional view of a linear motor system 130in accordance with an aspect of the present invention. The system 130includes a moving magnet assembly (or stage) 132 that is moveable in adirection of travel, indicated at 134, relative to a path 136. Forexample, the magnet assembly 132 is supported relative to the path 136for movement in the direction 134, such as by low or no frictionbearings (e.g., air bearings, not shown) to provide a desired air gapbetween the magnet assembly and the path.

The magnet assembly 132 includes a plurality of magnets 138, 140, 142,144, 146, 148, 150, and 152, which are attached to and extend from acommon side 156 of a back iron 158. An opposite side 160 of the backiron 158 is dimensioned and configured to conform to flux lines of amagnetic circuit formed between the magnet assembly and the path whenthe path is energized. That is, the thickness of the back iron 158between the opposed sides 156 and 160 is greater at locations betweenadjacent pairs of the magnets 138-152 than at locations generallyaligned with centers of the respective magnets. As a result, the sidesurface 160 has a generally scalloped or ribbed appearance between itsends; e.g., it is formed of alternating peaks and valleys between spacedapart ends of the back iron. The particular cross-sectionalconfiguration of the back iron can vary, such as described herein.

The path 136 includes a plurality of spaced apart teeth 162 that extendfrom a base portion 164 toward the magnet assembly 132 located above thepath 136. Typically, the teeth 162 are oriented substantially parallelrelative to each other and to the magnets 138-152. The path 136 alsoincludes windings 166 disposed around selected teeth. The windings 166could be pre-wound coil assemblies or wound in-situ around the teeth162.

Those skilled in the art will understand and appreciate that the linearmotor system typically includes a motor controller programmed and/orconfigured to control operation of the motor system 130. For example, anencoder or other positioning system provides the controller withposition information, based on which the controller controlsenergization of the associated windings 166 to effect desired movementof the magnet assembly 132 relative to the path. Those skilled in theart further will understand and appreciate various configurations ofpaths 136 and coil windings that could be utilized in combination with amagnet assembly in accordance with an aspect of the present invention.

FIG. 5 depicts a graphical representation of part of linear motor system200, similar to that shown in FIG. 4, illustrating magnetic flux lines202 for magnet circuits formed by a magnet assembly 204 and energizedwindings of a motor path 206 in accordance with an aspect of the presentinvention. The magnet assembly 204 includes a plurality of permanentmagnets 208, 210, 212, 214, 216, 218, 220, and 222 that are operativelycoupled to a back iron 226.

In accordance with an aspect of the present invention, as shown in FIG.5, the back iron 226 is dimensioned and configured to conform to themagnet flux lines that travel through the magnetic circuits formed ofthe magnet assembly and the path 206. The back iron 226 has a greatercross-sectional dimension at locations at ends of the magnet assembly204 and between adjacent pairs of magnets than at the locationsgenerally centered with the respective magnets. Consequently, theoverall mass of the moveable magnet assembly 204 is less than if suchportions had not been removed. Additionally, because the selectedportions have been removed according to the magnetic flux lines duringenergization of the path windings, the forces generated between theassembly 204 and the path remain substantially unchanged from a backplane that would include a substantially planar surface opposite themagnets.

To further reduce the mass of the magnet assembly, the magnets can beconfigured to have chamfered corners, so as to provide a generallyelongated octagonal geometry. The particular dimensions andconfiguration of magnets and back iron can be further optimized based onmagnetic finite element analysis. As a result, under Newton's law, theacceleration of the magnet assembly 204 relative to the path 206 isincreased by an amount proportional to the reduced mass of the magnetassembly. Additionally, the geometry further provides flux distributionthat is substantially sinusoidal distribution, which mitigates totalharmonic distortion (THD). A lower THD corresponds to a more efficientmotor.

As mentioned above, it is to be appreciated that various configurationsof magnet assemblies can be implemented in accordance with an aspect ofthe present invention. FIGS. 6-8 illustrate some examples of otherconfigurations of magnet assemblies that can be utilized. It will beunderstood and appreciated that such examples are solely forillustrative purposes and that numerous other possible configurationsexist, all of which are within the scope of the appended claims.

FIG. 6 illustrates a magnet assembly 240 for use in a linear motor inaccordance with an aspect of the present invention. The assembly 240includes a plurality of elongated permanent magnets 242 operativelycoupled to a back iron 244. As shown in FIG. 6, a cross-sectionaldimension of the back iron 244 varies along its length between spacedapart end portions 246 and 248 of the back iron. In particular, a sidesurface 250 of the back iron 244 opposite a side 252 to which themagnets 242 are attached has a substantially triangular or sawtoothgeometry having alternating peaks 254 and valleys 256. The other side252 is generally planar, although it includes slots or receptacles inwhich a portion of the respective magnets 242 is received. As a resultof such back iron 244 configuration, the thickness of the back iron atlocations between adjacent pairs of magnets 242 and at the end portions246 and 248 is greater than its thickness at locations generallycentered with the long axes of the respective magnets.

The geometry of the back iron 244 substantially conforms to magneticflux lines that travel through the back iron from the magnets so as toprovide extremely high flux densities. The geometry further enables theback iron 244 to have a reduced mass. The magnets also can be configuredto have chamfered corners, so as to provide a generally elongatedoctagonal geometry, such that the mass of the magnet assembly is furtherreduced. The combination of high flux densities and reduce back ironmass result in a high performance motor capable of achieving rapidacceleration compared to conventional linear motors of similar size.

FIG. 7 illustrates a magnet assembly 260 for use in a linear motor inaccordance with another aspect of the present invention. The assembly260 includes a plurality of elongated permanent magnets 262 operativelycoupled at a side surface 264 of a back iron 266. The back iron 266 hasa cross-sectional dimension that varies along its length between spacedapart end portions 268 and 270 in accordance with an aspect of thepresent invention. In particular, a side surface 272 of the back iron244 opposite the side 264 to which the magnets 262 are attached has aplurality of substantially elongated rectangular peaks (or protrusions)276. The peaks 276 extend between side edges of the back iron. That is,the side 272 has alternating rectangular peaks 276 and valleys 278 toprovide a generally square wave cross-sectional geometry between the endportions 268 and 270. The peaks 276 are generally centered over spacesbetween adjacent pairs of the magnets 262 and the valleys are generallycentered over the long axes of the respective magnets. The side 264 isgenerally planar, although it includes slots or receptacles in which aportion of the respective magnets 262 is received.

As a result of such geometry for the back iron 266, the thickness of theback iron at locations between adjacent pairs of magnets 262 and at theend portions 268 and 270 is greater than its thickness at locationsgenerally centered with the magnets. This geometry substantiallyconforms to magnetic flux lines that travel through the back iron 266from the magnets 262 so as to provide extremely high flux densities,such as when associated windings of a motor incorporating the magnetassembly 260 are energized. The geometry further enables the back iron266 to have a reduced mass. The combination of high flux densities andreduced back iron mass result in a high performance motor capable ofachieving rapid acceleration compared to conventional linear motors ofsimilar size.

FIG. 8 depicts yet another magnet assembly 280 in accordance with anaspect of the present invention. The magnet assembly includes aplurality of permanent magnets 282 operative connected to a generallyplanar side surface 284 of a back iron 286. Specifically, a portion ofthe magnets 282 can be received in associated slots or receptaclesformed in the side 284, although the magnets could be attached to theback iron in the absence of such slots.

In accordance with an aspect of the present invention, a side surface288 of the back iron 286 opposite the side 284 to which the magnets areconnected is dimensioned and configured to substantially conform tomagnetic flux lines associated with the magnet assembly when exposed tomagnetic fields from energized windings of an associated motor path (notshown). In the example of FIG. 8, the side surface 288 has alternatingpeaks 290 and valleys 292 to provide a generally sinusoidalcross-sectional configuration between spaced apart end portions 294 and296 of the back iron 286. The peaks 290 are generally centered overspaces located between adjacent pairs of magnets 282 and the valleys 292are generally centered over corresponding centers of the respectivemagnets.

As a result of the back iron geometry shown in FIG. 8, the magnetassembly 280 is able to complete magnetic circuits in an associatedlinear motor so as to provide extremely high flux densities, such aswhen associated windings of the motor incorporating the magnet assemblyare energized. The geometry further provides the back iron 286 with areduced mass. To further reduce the mass of the magnet assembly, themagnets 282 can be configured as a generally elongated octagon, such asby removing corner portions of the magnets. The combination of high fluxdensities and reduced back iron mass result in a high performance motorcapable of achieving rapid acceleration compared to conventional linearmotors of similar size.

What has been described above includes exemplary implementations of thepresent invention. It is, of course, not possible to describe everyconceivable combination of components or methodologies for purposes ofdescribing the present invention, but one of ordinary skill in the artwill recognize that many further combinations and permutations of thepresent invention are possible. For example, a magnet assembly, inaccordance with an aspect of the present invention can have differentnumbers of magnets from that shown and described herein. Additionally,the magnet assembly can have a different contour from the substantiallyflat configuration shown herein, such as to conform to the contour ofthe path with which the magnet assembly is to be utilized. Accordingly,the present invention is intended to embrace all such alterations,modifications and variations that fall within the spirit and scope ofthe appended claims.

1. A linear motor system, comprising: a path having a plurality of coilwindings that, when energized, provide a magnetic field; and a magnetassembly supported for movement in a direction of travel relative to thepath, the magnet assembly comprising: a plate having first and secondopposed surfaces that extend a length between spaced apart end portionsof the plate; an array of magnets operatively connected with the firstsurface of the plate along at least a substantial portion of the lengthof plate; and the plate having a cross-sectional dimension between thefirst and second surfaces that varies between the end portions so as tosubstantially conform to a magnetic field distribution when the magnetassembly is exposed to the magnetic field of the path.
 2. The system ofclaim 1, each of the magnets further comprising elongated generallyrectangular magnets having a generally elongated octagonal configurationand a long axis that extends between side edges of the plate.
 3. Thesystem of claim 1, the plate being formed of vanadium permeadur.
 4. Thesystem of claim 1, the plate having a greater cross-sectional dimensionbetween the first and second surfaces at locations between adjacentpairs of the magnets than at locations generally aligned with centers ofthe respective magnets.
 5. The system of claim 1, the second surface ofthe plate having a cross-sectional configuration that includes aplurality of alternating peaks and valleys between the end portions ofthe plate.
 6. The system of claim 5, at least a substantial number ofthe peaks being generally aligned with spaces located between adjacentpairs of the magnets.
 7. The system of claim 5, the valleys beinggenerally aligned with centers of respective magnets.
 8. The system ofclaim 7, each of the peaks and valleys extending between side edges ofthe plate and having a width extending between the first and second endsof the plate that is greater than or equal to zero inches and less thanone-half a width of an associated one of the magnets.
 9. The system ofclaim 1, the second surface of the plate having a generally triangularor saw tooth cross-sectional geometry.
 10. The system of claim 1, themagnet assembly further includes an outer encapsulation that engages theback iron in relationship with the array of magnets.
 11. The system ofclaim 10, the outer encapsulation being formed of a non-conductingmaterial.